1. Field of the Invention
The present invention relates a heat exchange unit, which comprises a plurality of heat transfer panels, which are formed of a metallic thin sheet and combined in parallel and integrally with each other, and especially to such a heat exchange unit in which heat exchange fluid having a high pressure can be introduced between the heat transfer panels.
2. Description of the Related Art
If there is a demand that heat transfer coefficient is increased to enhance heat exchange effectiveness, utilizing a heat exchanger by which heat exchange is made between a high temperature fluid and a low temperature fluid, a plate-type heat exchanger has conventionally been used widely. The plate-type heat exchanger has a structure in which a plurality of heat transfer panels having a plate-shape are placed parallelly one upon another at prescribed intervals so as to form passages, which are separated by means of the respective heat transfer panel. A high temperature fluid and a low temperature fluid flow alternately in the above-mentioned passages to make heat exchange through the respective heat transfer panels. Japanese Patent Provisional Publication No. S53-56748 describes an example of such a conventional plate-type heat exchanger.
In the conventional plate-type heat exchanger, gasket members formed of elastic material are placed between the adjacent two plates to make the distance between them constant and define passages for fluid. However, a high pressure of the heat exchange fluid flowing between the plates may cause deformation of the gasket member, thus disabling an appropriate separation of the fluids from being ensured or leading to an unfavorable variation in distance between the plates. In such a case, an effective heat exchange may not be carried out, thus causing a problem. In view of these facts, the conventional heat exchanger involves a problem that the heat exchange fluids can be utilized only in a pressure range in which the gasket member withstands.
There has recently been proposed a heat exchanger having a structure in which metallic thin plates, which are placed at predetermined intervals, are joined together, without using any gasket members, at their ends by welding to assemble the plates into a single unit so as to form passages for heat exchange fluids, on the opposite sides of the respective plates. Japanese Patent Provisional Publication No. 2003-194490 describes, as an example of an invention made by the present inventor, a heat exchange unit in which heat transfer panels formed of metallic thin plates are aligned in parallel with each other so as to be apart from each other, these plates are welded at their periphery excepting one side into a united body having an opening, and the opening is closed by an end plate.
Japanese Patent Provisional Publication Nos. S53-56748 and 2003-194490 describe the conventional heat exchanger (heat exchange unit). Especially, in the prior art described in the latter publication, a pair of heat transfer panels are welded together at flat portions on their opposite two sides, thus preparing a combined body section. The same step as mentioned above is repeated to prepare a plurality of combined body sections. A pair of combined body sections thus prepared are welded together at flat portions on their other opposite two sides and the same step as mentioned above is repeated to prepare a single combined body. Such a single combined body is inserted into an opening provided in the middle of an end plate, and then, the heat transfer plates and the end plates are welded together, thus ensuring a state in which openings formed on the respective sides of the heat transfer panels are separated from each other.
It is possible to weld the heat transfer panels together at their flat portions in an effective manner by mainly applying a seam welding. However, when a pair of combined body sections and another pair of combined body sections are welded together, a physical restriction that the distance between the combined body sections is relatively small makes it impossible for an electrode of the seam welding machine to reach the end of the flat portion, thus leading to difficulty in operation of the seam welding. There is no choice but to apply a different type of welding method to such a region to which the seam welding cannot be applied, thus causing problems that a general welding operation becomes time-consuming and strength of the welded portions utilizing the above-mentioned different type of welding method is deteriorated in comparison with the welded portion utilizing the seam welding method.
In addition, when one of objects to be welded has a relatively smaller thickness than the other as in case where the end plate and the heat transfer plates are welded together, it is difficult to automatically control the welding, since an excessively high heat may melt the smaller thickness object unless a severe control of generated heat is made. Further, there is need to connect water-tightly the end plate to the heat transfer panels over serrations of the end plate, which are placed along the heat transfer panels, as well as portions of the end plate, which define the opening for the single combined body and have a complicated shape, so as to keep the fluid openings of the heat transfer panels in a properly separated state. In view of such circumstances, there has been no choice but to apply a fillet welding as the welding method for such a structure. However, there is need to carry out carefully a welding operation along portions to be welded, having complicated shapes, resulting in a low operation efficiency. In addition, there is also a problem that the resultant welded structure does not have a sufficient strength against pressure of heat exchange fluids. There is a further problem that sputters, which come from the welded portion into the fluid opening of the heat transfer panels during the welding operation, are deposited on the surface of the heat transfer panel and may be left as particles that freely move within the space between the heat transfer panels.
In case where liquid having a high metallic corrosion property such as seawater is used as heat exchange fluid, titanium is used as material for forming the heat transfer panels. A gas-shielded arc welding is applied to weld such panels made of titanium, due to its high reactivity. There is limitation in heat generated in welding of the end plate and the heat transfer panels, as mentioned above, with the result that there is no choice but to apply a TIG welding in which a precise control of generated heat can be made. However, there are problems that a welding speed is low, a gap between objects to be welded is required to be small in order to enhance welding precision for the objects to be welded, thus causing many limitations in welding, and a general welding operation becomes time and cost-consuming.